Method of manufacturing micro-disperse particles of sodium borohydride

ABSTRACT

A compact solid source of hydrogen gas, where the gas is generated by contacting water with micro-disperse particles of sodium borohydride in the presence of a catalyst, such as cobalt or ruthenium. The micro-disperse particles can have a substantially uniform diameter of 1-10 microns, and preferably about 3-5 microns. Ruthenium or cobalt catalytic nanoparticles can be incorporated in the micro-disperse particles of sodium borohydride, which allows a rapid and complete reaction to occur without the problems associated with caking and scaling of the surface by the reactant product sodium metaborate. A closed loop water management system can be used to recycle wastewater from a PEM fuel cell to supply water for reacting with the micro-disperse particles of sodium borohydride in a compact hydrogen gas generator. Capillary forces can wick water from a water reservoir into a packed bed of micro-disperse fuel particles, eliminating the need for using an active pump.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional application No.60/349,015 “A Novel Fuel Approach for Hydrogen Generation from Solids”,by Kravitz, et al., filed Jan. 15, 2002, which is incorporated herein byreference.

This application is a divisional application of Ser. No. 10/830,989filed Apr. 23, 2004 now U.S. Pat. No. 7,306,780; which in turn is adivisional application of allowed U.S. patent application Ser. No.10/191,900, “Compact Solid Source of Hydrogen Gas”, by Stanley H.Kravitz, et al., filed Jul. 9, 2002, now U.S. Pat. No. 6,746,496, whichare herein incorporated by reference.

FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant toDepartment of Energy Contract No. DE-AC04-94AL85000 with SandiaCorporation.

BACKGROUND OF THE INVENTION

The present invention relates generally to methods and materials for thegeneration of hydrogen gas from solid hydrogen storage materials. Inparticular, the present invention relates to the generation of hydrogengas by contacting water with micro-disperse particles of sodiumborohydride in the presence of a catalyst, such as cobalt or ruthenium.

Hydrogen gas is used as a fuel for fuel cells, as a purge gas inchromatographs, and for combustors. Micro fuel cells, micro gaschromatographs (i.e., micro chem labs-on-a-chip), and micro-combustorsall require a compact, high-density, controllable source of hydrogengas. Gas cylinders are too heavy and bulky, while liquid hydrogenrequires cyrogenic cooling. Metal hydride systems are limited to 1-3%hydrogen by weight; are endothermic (that is, as hydrogen is evolved,the container gets colder, which reduces the hydrogen vapor pressure);the hydrogen evolution rate is not controllable or adjustable (so thatan oversized amount of hydride is necessary); and the metal hydrides arequite expensive.

Micropower is the key to making integrated microsystems. When a compacthydrogen source is developed to compliment a micro fuel cell, or feed agas chromatograph, a total system package will be available for makingpower or purge gas, that currently is provided by large batteries orlarge gas cylinders.

Metal hydrogen complexes, such as sodium borohydride (NaBH₄), zincborohydride (ZnBH₄), potassium borohydride (KBH₄), calcium borohydride(CaBH₄), lithium aluminum hydride (LiAlH₄), sodium boron trimethoxyhydride (NaBH(OCH₃)₃), and so on, are attractive solid sources ofhydrogen. When reacted with water, in the presence of a suitablecatalyst, these metal hydrogen complexes can provide a hydrogen gasyield from 11-14% by weight (which is 5-6 times more hydrogen releasedper gram than for metal hydrides).

Sodium borohydride (also known as sodium tetrahydridoborate) is aparticularly attractive solid source of hydrogen since its equivalentenergy density is nearly equal to that of diesel fuel. It is commonlyused in a variety of industrial processes (e.g., as a bleaching agent inpaper and newsprint production and recycling). As an extremely powerfulreducing agent, it is also used to reduce impurities in the chemicalsprocessing industry, and to reduce metals from industrial waste streamsand effluents (e.g., recovering copper from printed circuit boardwastewater).

Sodium borohydride reacts exothermically with water in the presence of acatalyst (or when acidified) to produce hydrogen gas and sodiummetaborate (i.e., Borax) according to the following reaction:NaBH₄+2H₂O→NaBO₂+4H₂+Heat(300 kJ) (catalyst)  (1)This reaction is particularly efficient at generating hydrogen gas,since the sodium borohydride supplies two of the hydrogen gas molecules(H₂), and the water supplies the other two molecules, for a total offour molecules of H₂. The reaction is exothermic; does not require theaddition of heat or the use of high pressure to initiate; and cangenerate hydrogen even at 0 degrees C.

Solid sodium borohydride is a particularly attractive choice as acompact and high-density source of hydrogen fuel for fueling a ProtonExchange Membrane (PEM) hydrogen/oxygen fuel cell (i.e., PEM fuel cell).

Dry sodium borohydride is conventionally produced as a powder (i.e.,particles) having particle sizes in the range of 100-600 microns. Ananti-caking/flow agent additive is typically added immediately afterremoval of the borohydride powder from a vacuum dryer to promotefree-flow of the powder (unless the dried powder is immediatelycompacted into a compacted product form). See U.S. Pat. No. 5,182,046 toPatton, et al.). Examples of compacted product forms include 25 mmdiameter×6 mm pellets, 5×11×17 mm caplets, and granules. A typicalanti-caking agent comprises 0.5% by weight of silica or magnesiumcarbonate nanoparticles. Here, the word “caking” means the uncontrolledagglomeration and/or aggregation of individual fuel particles into alarger mass (i.e., “cake”).

Alternatively, a fluidized bed dryer can be used instead of a vacuumdryer, which produces free-flowing particles of solid sodium borohydridewithout the need for using anti-caking or flow additives. See U.S. Pat.No. 6,231,825 to Colby, et al. This is because the fluidized bed processproduces particles having a significantly larger average particle size(600 microns), as compared to the average size of vacuum dried particles(100-200 microns). Sodium borohydride is produced commercially by Mortonand Eagle-Picher in the USA; by Finnish Chemicals (Nokia) in Finland;and Bayer in Germany.

Adding water to commercially available powders, granules, caplets, orpellets of solid sodium borohydride (in the presence of a suitablecatalyst, such as cobalt or ruthenium) results in caking and scaling ofthe borohydride surface due to production of the reactant product sodiummetaborate (NaBO₂) in the form of a surface layer (i.e., crust orscale). As the layer of scale grows progressively thicker, the water hasa progressively harder time penetrating through the metaborate crust toreach the unreacted NaBH₄ fuel below, resulting in a decreased hydrogenproduction rate (it can even stop producing gas if the scale is thickenough). Note that this is a different phenomenon than the previousproblem of “caking” caused by agglomeration of fine powders that occursduring production (that was described earlier).

The problem of scale/crust formation during hydrogen generation can beprevented by using a dilute aqueous solution of NaBH₄. In thiswell-known approach, sodium borohydride powder is dissolved into waterstabilized with 1-10% NaOH or KOH. The alkaline state of the solutionprevents the dissolved sodium borohydride from decomposing andprematurely releasing hydrogen gas. The solubility limit of sodiumborohydride in water at room temperature is 44 wt % (and decreases atlower temperatures). An example of a commercially available stabilizedaqueous solution called VenPure® is available from Rohm and Haas, Inc.that comprises 12% NaBH₄, 40% NaOH, and the balance water.

When using a dilute aqueous solution of NaBH₄, hydrogen gas is generatedby contacting the solution with a metal catalyst. For example, in U.S.Pat. No. 6,3587,488 (which is herein incorporated by reference), Sudadiscloses a method of generating hydrogen gas by adding small amounts ofpowdered catalysts, such as cobalt, nickel, or Mg₂Ni (fluorinated orunfluorinated) to a stabilized, alkaline solution containing 10% sodiumborohydride.

Likewise, the Hydrogen-on-Demand™ system from Millennium Cell, Inc. usesa liquid fuel source comprising a 20-30% aqueous solution of NaBH₄stabilized with 1-3% NaOH. In their system, a fuel pump and valvesdirects liquid fuel from a storage tank containing 20-30% sodiumborohydride solution into a catalyst chamber (e.g., ruthenium spongemetal). Upon contacting the catalyst bed, the fuel solution generateshydrogen gas and sodium metaborate (in solution). The hydrogen gas andmetaborate solution are separated in a second chamber, and themetaborate solution is stored as a waste product in a collection tank.The heat generated during the reaction is sufficient to vaporize some ofthe water present. As a result, the hydrogen gas is supplied at 100%relative humidity to the PEM fuel cell. The hydrogen gas can beoptionally processed through a heat exchanger to achieve a specifiedlevel of humidity before being sent to the PEM fuel cell forconsumption. This Hydrogen-on-Demand™ system has been successfullydemonstrated to electrically power a Chrysler Town & Country Minivan.

Disadvantages of this approach include the relatively low energy densityof the diluted liquid fuel (e.g., 10-30% NaBH₄), which makes it onlyslightly better than metal hydrides. Additionally, a pump is requiredfor circulating the liquid fuel, which causes a parasitic drain on thenet power production from the fuel cell.

These disadvantages become particularly severe if liquid sodiumborohydride is used for micro-sized PEM fuel cells, where the goal is tominiaturize every component in the system, while retaining highefficiency of fuel usage and power generation. The system requirementsfor a micro fuel cell or a micro gas chromatograph are considerablydifferent than for a Minivan.

What is needed, therefore, is a material and system for generatinghydrogen gas that utilizes solid sodium borohydride in a highlyefficient manner that prevents caking and scaling from reducing thehydrogen production yield, and preferably, without using a fuel pump.

Such a device should provide 5-6 times more hydrogen than existingsources, with the additional possibilities of integration with silicondevices for sensing, control, and MEMS functions. This device should beable to be directly integrated with micro fuel cell designs to create avery compact micro power system. This device should also be scalable tolarger systems needing larger amounts of hydrogen for higher powerapplications.

Against this background, the present invention was developed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification, illustrate various examples of the present inventionand, together with the detailed description, serve to explain theprinciples of the invention.

FIG. 1 shows a schematic block diagram of a system for producingelectricity using a PEM fuel cell, including a closed loop wastewatermanagement system, according to the present invention.

FIG. 2 shows a schematic isometric view of a micro hydrogen gasgenerator, according to the present invention.

SUMMARY OF THE INVENTION

The present invention relates to the generation of hydrogen gas bycontacting water with micro-disperse particles of sodium borohydride inthe presence of a catalyst, such as cobalt or ruthenium. Themicro-disperse particles can have a substantially uniform diameter inthe range of about 1-10 microns, and preferably about 3-5 microns.Ruthenium or cobalt catalytic nanoparticles can be incorporated in themicro-disperse particles of sodium borohydride, which allows a rapid andcomplete reaction to occur without the problems associated with cakingand scaling of the surface by the reactant product sodium metaborate. Aclosed loop water management system can be used to recycle wastewaterfrom a PEM fuel cell to supply water for reacting with themicro-disperse particles of sodium borohydride in a compact hydrogen gasgenerator. Capillary forces can wick water from a water reservoir into apacked bed of micro-disperse fuel particles, eliminating the need forusing an active pump.

DETAILED DESCRIPTION OF THE INVENTION

The present invention solves the problem of crust formation (i.e.,caking, scaling) of the surface of solid sodium borohydride particles,granules, caplets, or pellets from the reaction product sodiummetaborate (i.e., borax) during hydrogen production by usingmicro-disperse particles of solid sodium borohydride. The micro-disperseparticles can be in the form of microspheres having a substantiallyuniform diameter less than 100 microns. Alternatively, themicro-disperse particles can have a substantially uniform diameter inthe range of about 1-10 microns, and preferably 3-5 microns. The phrase“micro-disperse” means that the fuel particles are in the 1-100 micronsize range, and are non-agglomerated (i.e., dispersed).

The fuel particles can be packed in a simple cubic pattern. About 52% ofthe volume would be solid, with the other 48% being empty space (i.e.,interstices or pores). These interstices allow water, either in the formof liquid water, a fine mist of water, or water vapor, to flow throughand make intimate contact the with surfaces of the NaBH₄ microspheres.Because about ½ of the total volume is in the form ofsubstantially-interconnected porosity, the water can easily flow betweenthe micro-disperse particles. The micron-sized particles have a verylarge effective surface area (per unit volume) available for reactingwith the water.

By using micro-disperse fuel particles, the water only has to diffusethrough a very thin layer of sodium metaborate to totally react eachfuel particle to completion. Such a thin layer of crust should bepermeable to water. In this sense, the diameter of the fuel particle canbe chosen to be smaller than a threshold diameter where formation of asodium metaborate crust would significantly reduce the hydrogenproduction rate for particles that have a diameter greater than thisthreshold diameter. The meaning of a significant reduction in thehydrogen production rate can be defined as a loss greater than 20%.Therefore, this approach significantly reduces, and likely eliminates,the problem of reduced hydrogen production yield caused by caking andscaling.

The metal catalyst can be incorporated into each fuel particle ofmicro-disperse sodium borohydride. By “incorporated”, we mean that themetal catalyst can disposed in the center of the particle (i.e., as a“seed”), or dispersed throughout the bulk of the particle (eitheruniformly or non-uniformly), or placed as a coating on the surface ofthe particle, or any combination thereof. The word “incorporated” alsorefers to a two-component fuel mixture, where one component is thesodium borohydride particle (without catalyst), and the other componentis the metal catalyst nanoparticle; and where the two-components aremechanically mixed together to make the fuel mixture having a catalystincorporated therein.

The use of the word “microsphere” is defined as includingirregularly-shaped, oval-shaped, and oblong-shaped particles, having acharacteristic, equivalent “diameter”.

Because the gap between individual fuel particles is so small (1-10microns) in a closely-packed array, fluid capillary forces can pull(i.e., wick) liquid water into, and through, a packed bed ofmicro-disperse fuel particles. This eliminates the need for using anactive pump in micro fuel cells, micro gas chromatographs, etc. All thatis required is a valve for controlling the follow of water to the bed offuel particles.

The micro-disperse particles (i.e., crystals) of solid sodiumborohydride can be produced a variety of ways. One method is to dissolvethe sodium borohydride in Diethylene Glycol Dimethyl Ether (diglyme) atelevated temperature. Above 80 C the sodium borohydride is soluble indiglyme. Micron-sized, mono-disperse particles of sodium borohydride cansubsequently be re-precipitated by rapidly cooling the heated,solubilized solution. This can be accomplished, for example, by using anozzle to spray the solubilized solution into a room temperature bath ofdiglyme. Optionally, a nucleating agent can be added to help nucleatethe particles. In particular, the nucleating agent can be a suitablecatalytic metal, such as cobalt, ruthenium, nickel, etc, as is well-knowto those of ordinary skill in the art. In this way, a small amount ofcatalyst is incorporated into each micro-disperse fuel particle. Then,the re-precipitated micro-disperse particles can be separated from thecold diglyme solution by using vacuum filtration with, for example, a0.5 micron filter.

In general, processing should be performed in an inert environment(e.g., under nitrogen gas, or using a nitrogen gas bubbler), usingnon-metallic reactors, stirrers, etc. (e.g., glass, Teflon), to preventany possible catalytic reaction with metal impurities that could lead touncontrolled generation of hydrogen gas, causing a safety issue.

If agglomeration of these micron-sized particles becomes a problem, thennanoparticles of silica or magnesium carbonate can be added to preventagglomeration.

Optionally, the catalytic agent can be added as a surface component tothe micro-disperse fuel particle at a later date. Alternatively, thecatalyst can be mixed in the form of small particles along with themicro-disperse particles of sodium borohydride to form a two-componentmixture. Ruthenium nanoparticles can be used for the catalyst. Thecobalt catalyst can be derived from cobalt chloride. The rutheniumcatalyst nanoparticles can be suspended in alcohol, and then mixed withthe re-precipitated solution of sodium borohydride in diglyme to coatthe surface of the fuel particles (NBH₄) with a thin layer of thecatalyst.

The micron-sized fuel particles produced by these methods generallyexhibit Brownian motion and behave as a colloidal system. Depending onthe surface charge state, the particles may (or may not) stick together.A variety of well-known surface treatments (e.g., nanoparticles ofsilica or magnesium carbonate) can be used to adjust the surface chargeto control the degree of sticking (or non-sticking), required forefficient handling of the particles. In general, the micron-sized fuelparticles will form a network with high permeability to fluid flow(e.g., liquid water, water vapor) when loaded into a powder bed (i.e.,fuel reservoir). Such a porous structure readily allows for theevolution of hydrogen gas during the flow of water. However, the benefitof using a more tightly packed bed needs to weighed against thedifficulty of permeating/transporting water through the bed, either bycapillary action, or by forced flow. The packed bed needs to besufficiently porous so that formation of the reactant product sodiummetaborate doesn't clog up the bed and choke off water flow. Bubbling ofthe hydrogen gas as it is produced may help to loosen the bed and keepit sufficiently porous.

If a pump is used, then pumping water vapor instead of liquid waterwould allow for a smaller, lower power pump to be used because the lowviscosity of water vapor makes it easier to pump than liquid water. Apump can be used as part of a control system, i.e., speeding up whenhigher power production is required, etc.

The combination of micro-disperse particles with water vapor pumping canprovide a high efficiency, high yield source of water vapor-saturatedhydrogen gas, controlled by a small, efficient gas pump and controlsystem.

FIG. 1 shows a schematic illustration of a power system for producingelectricity using a PEM fuel cell, including a closed loop wastewatermanagement system, according to the present invention. The power systemcomprises a compact hydrogen gas generator 10, comprising a supply ofmicro-disperse particles of sodium borohydride mixed with a catalystselected from the group consisting of ruthenium and cobalt; a ProtonExchange Membrane (PEM) fuel cell 20 for combining hydrogen gas suppliedby module 10 with oxygen supplied by air to produce electricity andwastewater; a water storage unit 30; a water pump 40; fluid transfermeans 50 for carrying water from PEM fuel cell 20 to the water storageunit 30, then to water pump 40, and then to hydrogen generator 10,whereupon the water contacts the supply of micro-disperse particles ofsodium borohydride mixed with a catalyst and generates hydrogen gas; gastransfer means 60 for carrying hydrogen gas from hydrogen generator 10to the PEM fuel cell 20; means 70 for introducing air (i.e., oxygen) tothe PEM fuel cell; means 80 for removing electrons from PEM fuel cell20; and pump controller 90 operatively associated with water pump 40 forcontrolling its operation.

The example shown in FIG. 1 illustrates a closed loop water managementsystem, which allows a sodium borohydride fuel cell system to have ahigh energy density. In the borohydride system, one mole of water isrequired to create two moles of molecular hydrogen. PEM fuel cell 20produces one mole of water per mole of hydrogen. A sodium borohydridewater management system recovers and recycles wastewater from the fuelcell cathode, and stores it. The stored water is then pumped onto solidsodium borohydride that was mixed with a suitable catalyst, such ascobalt. Hydrogen gas is created in module 10, with half originating fromthe sodium borohydride and the other half from the fuel cell wastewaterstream, so that the system is not a net consumer of water. This allowsthe borohydride to be stored as a solid, with an energy density nearlythat of diesel fuel.

FIG. 2 shows a schematic isometric view of an example of a microhydrogen gas generator, according to the present invention. Gasgenerator 100 comprises a substantially planar substrate 110 with a pairof cavities (i.e., reservoirs) recessed into the substrate, i.e., waterreservoir 120 and fuel reservoir 160. Flow channel 150 fluidicallyconnects water reservoir 120 to fuel reservoir 160. A plurality ofcapillary lines 130 collect water from water reservoir 120 and feedwater to the proximal end of flow channel 150. Then, capillary lines 135located at the distal end of flow channel 150 distribute the water fromflow channel 150 to a plurality of locations at the proximal end fuelreservoir 160. Fuel reservoir 160 holds a packed bed of micro-dispersefuel particles 170. Micro-disperse fuel particles 170 can comprisesodium borohydride with an incorporated catalyst. Micro-disperse fuelparticles 170 can have a substantially uniform diameter from 1-10microns, and, preferably, from about 3-5 microns. The catalyst can becobalt, ruthenium, or nickel. Fuel particles 170 can be packed insidefuel reservoir 160 in the form of a hexagonal close-packed, a cubicarray, or a network of aggregated particles having fractal dimensions.Because the dimensions of the open space in-between closely-packed fuelparticles 170 is very small (i.e., less than 10 microns), capillaryforces can pull (i.e., wick) water completely into fuel reservoir 160.Hence, no active pump is required to bring water from water reservoir120 to fuel reservoir 160.

Valve 140 controls the flow of water from water reservoir 120 to fuelreservoir 160. The valve could be controlled by the current demands ofthe micro fuel cell. When the current demands are high, the flow ofhydrogen must be high and the water valve would be fully open. Atstandby, the valve would be nearly closed, limiting the flow of waterand the production of hydrogen.

A wide variety of designs can be used for valve 140. Valve 140 can be aMEMS structure fabricated using surface micromachining techniques. Valve140 can be a normally-closed bimetallic or bimorph strip. Resistanceheating or piezoelectric force (using electricity produced by anadjacent micro fuel cell) would activate and open the valve. Valve 140in its normally-closed position would push on the top of a thin siliconemembrane and restrict the flow of water in channel 150. This valve woulduse little energy from the micro fuel cell during hydrogen demand, andno energy at standby.

An alternative to the bimetallic/bimorph valve would be a normally-openvalve, also with a thin top wall, and having a pressure chamber over thechannel. This chamber could fill with hydrogen as the reaction proceeds.When hydrogen usage dropped, the chamber would pressurize and close thevalve to the water supply. When hydrogen demand returns, the chamberpressure would drop and the valve would open allowing water to enter.This scheme uses no power, but does require a package and valve capableof holding pressurized hydrogen.

A solid cap or cover lid (not shown) can be sealed to the upper surfaceof substrate 110 to complete packaging of micro hydrogen gas generator100. The cover lid can have openings for supplying water, and forremoving hydrogen gas. The gas outlet openings may need to be smallerthan the size of the fuel particles 170 so that fuel particles aren'tcarried out though the openings. Hence, the cover lid may need to bemade with a large area having engineered porosity (i.e., a screen) toallow the hydrogen gas to pass through, without letting fuel particlesescape. It is well-known in the art of surface micromachining to makeuniform grids of pores (circles, squares, oblongs, etc.) in silicon,silicon nitride, even metals, using microlithography and etchingtechniques or femtosecond lasers, with engineered openings as small as 1micron.

In a micro PEM fuel cell application, the Proton Exchange Membrane (PEM)can be in direct contact with (or with a small gap) the hydrogen gasevolving inside the fuel reservoir (i.e., the PEM becomes the cover lidover the fuel reservoir), so that the hydrogen gas doesn't need to betransported through pipes or channels to another location. Wastewatermade by an integrated micro PEM fuel cell can be recycled through shortmicrofluidic channels back to the water reservoir 120 for reuse andrecycle.

It is important to note that the hydrogen gas generated by generator 100has essentially 100% relative humidity (i.e., water vapor saturated),which is beneficial to the operation of a PEM fuel cell.

Substrate 110 should be made of a material that doesn't react withsodium borohydride, the catalyst, or the reaction products (hydrogen,sodium metaborate). Monocrystalline silicon, polysilicon, and siliconnitride can be used for substrate 110. Silicon surfaces can be coatedwith oxide where it needs to be hydrophilic, such as liquid channels130, 135, and 150; while other areas can be hydrophobic. Reservoirs 120and 160 can be manufactured by bulk micromachining methods. The overalldimensions of micro gas generator 100 can be, for example, 0.5 cm wide×2cm long×400 microns deep. Capillary lines 130 and 135 can be about20-100 microns wide and deep, sufficient to generate significantcapillary forces. The cover lid can be made of a borosilicate glass(e.g., Pyrex™), which can be anodically bonded to a substrate made ofsilicon. About 1 cc of water can be stored on board in a collapsiblecontainer so it could be easily pumped out. The size of water reservoir120 and fuel reservoir 160 can be easily scaled up or down to any size,consistent with the power requirements and fabrication limitations.

Optionally, fuel reservoir 160 can be filled with a two-component fuelmixture comprising micro-disperse particles of sodium borohydride andunincorporated particles of a suitable metal catalyst.

The interior volume of fuel reservoir 160 may need to be significantlyoversized to allow for expansion of the packed fuel bed to prevent anyswelling of the bed, and/or buildup of the reaction product sodiummetaborate, from clogging up and choking off the flow of water throughthe bed.

FIG. 2 illustrates an example of a 13.8% compact, solid-state, microhydrogen gas generator with a controllable valve, where the valve can becontrolled by the current demands of an adjacent fuel cell. Inparticular, the design illustrated in FIG. 2 can be used in conjunctionwith the micro fuel cell described in co-pending U.S. patentapplication, “Fuel Cell and Membrane”, Ser. No. 10/056,736 to Klitsner,et al., which is incorporated herein by reference.

Other embodiments of a micro hydrogen gas generator, like the exampleshown in FIG. 2, can be manufactured. For example, a MEMS micro-pump canbe added to force water to flow from water reservoir 120 to fuelreservoir 160. However, the benefit of having greater flow and, hence,greater hydrogen production rate needs to be balanced against the energyrequired to power the pump.

Alternatively, the bimetallic/bimorph valve in the example shown in FIG.2 can be eliminated. In this case, the addition of water into waterreservoir 120 would initiate the chemical reaction, which would producehydrogen gas in an essentially uncontrolled and continuous manner untileither the source of water or fuel runs out. Such an application mighthave use in a one-time power source, such as for an emergencytransmitter, where there is no demand or requirement for having controlof the output from the micro fuel cell.

In another embodiment, with reference to the apparatus shown in FIG. 2,water reservoir 120 is filled with water comprising nanoparticles of themetal catalyst suspended therein. In this case, fuel particles 170 wouldnot be required to have a metal catalyst incorporated therein. Methodsto suspend nanoparticles in water are well-known in the art, and caninclude use of a polymer additive to stabilize the solution and preventaggregation/coagulation.

The particular examples discussed above are cited to illustrateparticular embodiments of the invention. Other applications andembodiments of the apparatus and method of the present invention willbecome evident to those skilled in the art.

The actual scope of the invention is defined by the claims appendedhereto.

1. A method of manufacturing micro-disperse particles of sodiumborohydride, comprising: dissolving sodium borohydride in diglyme at atemperature greater than or equal to 80 C to produce a heated,solubilized solution; precipitating substantially uniform particles ofsodium borohydride by rapidly cooling the heated, solubilized solution;and separating out the precipitated particles by microfiltering therapidly cooled solution, whereby micro-disperse particles of sodiumborohydride are produced.
 2. The method of claim 1, wherein rapidlycooling comprises spraying the heated, solubilized solution through anozzle into a cold bath of diglyme.
 3. The method of claim 1, furthercomprising adding a nucleation agent to the heated, solubilized solutionof sodium borohydride dissolved in diglyme; wherein the nucleation agentcomprises nanoparticles of a catalytic metal selected from the groupconsisting of cobalt, ruthenium, nickel, and copper; whereby thecatalyst becomes incorporated into the micro-disperse particles ofsodium borohydride.
 4. The method of claim 1, further comprising addingthe catalytic agent as a surface component to the micro-disperseparticles of sodium borohydride, whereby the catalyst becomesincorporated into the micro-disperse particles of sodium borohydride.